Since the invention of the integrated circuit (IC), semiconductor chip features have become exponentially smaller and the number of transistors per device exponentially larger. Advanced IC's with hundreds of millions of transistors at feature sizes of 0.25 micron, 0.18 micron, and less are becoming routine. Improvement in overlay tolerances in optical photolithography, and the introduction of new light sources with progressively shorter wavelengths, have allowed optical steppers to significantly reduce the resolution limit for semiconductor fabrication far beyond one micron. To continue to make chip features smaller, and increase the transistor density of semiconductor devices, IC's have begun to be manufactured that have features smaller than the lithographic wavelength.
Sub-wavelength lithography, however, places large burdens on optical lithographic processes. Resolution of anything smaller than a wavelength is generally quite difficult. Pattern fidelity can deteriorate dramatically in sub-wavelength lithography. Critical dimensions (CD's), which are the geometries and spacings used to monitor the pattern size and ensure that it is within the customer's specification, are especially important to have size maintenance during processing. Semiconductor features may deviate significantly in size and shape from the ideal pattern drawn by the circuit designer. These distortions include line-width variations dependent on pattern density, which affect a device's speed of operation, and line-end shortening, which can break connections to contacts. To avoid these and other optical proximity effects, the semiconductor industry has attempted to compensate for them in the photomasks themselves.
This compensation is generally referred to as optical proximity correction (OPC). The goal of OPC is to produce smaller features in an IC using a given equipment set by enhancing the printability of a wafer pattern. OPC applies systematic changes to mask geometries to compensate for the nonlinear distortions caused by optical diffraction and resist process effects. A mask incorporating OPC is thus a system that negates undesirable distortion effects during pattern transfer. OPC works by making small changes to the IC layout that anticipate the distortions. OPC offers basic corrections and a useful amount of device yield improvement, and enables significant savings by extending the lifetime of existing lithography equipment. Distortions that can be corrected by OPC include line-end shortening, corner rounding, isolated-dense proximity effect, and isolated-line depth of focus reduction.
Another difficulty with sub-wavelength photolithography is that, as two mask patterns get closer together, diffraction problems occur. At some point, the normal diffraction of the exposure rays start touching, leaving the patterns unresolved in the resist. The blending of the two diffraction patterns into one results from all the rays being in the same phase. Phase is a term that relates to the relative positions of a wave's peaks and valleys. One way to prevent the diffraction patterns from affecting two adjacent mask patterns is to cover one of the openings with a transparent layer that shifts one of the sets of exposing rays out of phase, which in turn nulls the blending.
This is accomplished by using a special type of photomask known as a phase shift mask (PSM). A typical photomask affects only one of the properties of light, the intensity. Where there is chromium, which is an opaque region, an intensity of zero percent results, whereas where the chromium has been removed, such that there is a clear or transparent region, an intensity of substantially 100 percent results. By comparison, a PSM not only changes the intensity of the light passing through, but its phase as well. By changing the phase of the light by 180 degrees in some areas, the PSM takes advantage of how the original light wave adds to the 180-degree wave to produce zero intensity as a result of destructive interference.
Another particular issue that impacts the quality of optical lithography is focus variation, which is nearly ubiquitous in IC manufacturing because of the combined effects of many problems, such as wafer non-flatness, auto-focus errors, leveling errors, lens heating, and so on. A useful optical exposure process should be able to print acceptable patterns in the presence of some focus variation. Similarly, a useful optical exposure process should be able to print acceptable patterns in the presence of variation in the exposure dose, or energy, of the light source being used. To account for these simultaneous variations of exposure dose and focus (or lack thereof), it is useful to map out the process window, such as an exposure-defocus (ED) window, within which acceptable lithographic quality occurs. The process window for a given feature shows the ranges of exposure dose and depth of focus (DOF) that permit acceptable quality.
All of these considerations make for difficult optical photolithographic processing. Utilizing OPC in photomasks, and employing PSM photomasks, renders the resulting photomasks complicated, increasing the mask error factor (MEF) of such photomasks. Poor resolution and DOF results in a small process window in which to perform traditional optical lithography. Thus, utilizing traditional optical photolithography with complicated photomasks for semiconductor fabrication has become difficult, even though optical photolithography is a desired process in that it is a quick process.
One solution is to write on the semiconductor wafer directly, using an electron beam, or e-beam, instead of using traditional optical exposure. E-beam lithography, as it is known uses an electron source that produces a small diameter spot, or shot, and a blanker capable of turning the beam on and off. The exposure takes place in a vacuum to prevent air molecules from interfering with the electron beam. The beam passes through electrostatic plates capable of directing, or steering, the beam in the x and y directions on the wafer. Precise direction of the beam usually requires that the beam travel in a vacuum camber in which there is the electron beam source, support mechanisms, and the substrate being exposed.
Since a computer generates the desired pattern, the beam is directed to specific positions on the wafer surface by a deflection subsystem, and the beam turned on where the resist is to be exposed. Larger substrates are mounted on an x-y stage and are moved under the beam to achieve full surface exposure. This alignment and exposure technique is referred to as direct writing. The pattern is exposed in the mask by either raster or vector scanning. In the former, a computer directs the movement and activates the blanker in desired regions. A drawback to raster scanning is the time required for the beam to scan, since it travels over the entire surface of the wafer. By comparison, in vector scanning, the beam is moved directly to the regions that have to be exposed. At each position, small square- or rectangular-shaped areas are exposed, building up the desired shape of the exposed area.
E-beam direct writing overcomes some of the problems associated with optical photolithography. It generally avoids the small process window of conventional optical photolithography. Furthermore, it generally allows for highly precise CD's, that otherwise would require complex optical photomasks that employ OPC, and that are PSM's. However, e-beam writing has the unfortunate disadvantage that it is significantly slower than conventional optical photolithography. For example, e-beam direct writing may be about ten times slower than optical exposure techniques.
Therefore, there is a need photolithography that overcomes these disadvantages in the manufacture of photomasks. Specifically, there is a need for photolithography that allows for highly precise CD's, without the difficulty of optical exposure techniques. Such photolithography should further be faster than traditional e-beam direct writing. For these and other reasons, there is a need for the present invention.